Overheating is very trick to diagnose, because so many systems are inter-related. So you need to diagnose very logicall, and step by step.
Two possibilities are the most obvious.
1. The electric cooling fans are not working
2. The thermostat is stuck completely in the shut position
Both of these can be tested...the fans visually and aurally, the thermostat by removing it and placing it in a pan of hot water and seeing what it does, or does not, do.
If the fans and thermostat pass the test, then we go to other ideas:
1. Was the coolant replaced properly, that is, not leaving any huge air pockets in the cooling system?
2. Is this a very high mileage car, where it is possible that the radiator could be clogged up? Can you look in the radiator with a light, and see lots of crud built up?
If the coolant has been filled properlly and the radiator appears to be clean inside, or has been flow-tested, then we go to more drastic things.
1. Pressurize the cooling system, and while under pressure, extract the spark plugs one by one. Look for coolant on the tips of the plugs. If you see any, you have a head gasket leak.